1. Field of the Invention
This invention relates to a coating solution for forming a film for cutting off solar heat radiation on a sheet of glass or a transparent resin, or other material forming a window of a vehicle, building, office or house, a shopwindow, a lamp, etc., and the film formed therefrom.
2. Description of the Prior Art
It has hitherto been usual to employ an inorganic material or resin reflecting visible light and infrared radiation for forming a film for eliminating or reducing the generating range of solar radiation heat. Typical examples of the inorganic materials which have been employed are a metal oxide, such as FeO.sub.x, CoO.sub.x, CrO.sub.x, or TiO.sub.x, and a metal having a large quantity of free elections, such as Ag, Au or Al. A typical resin film has been formed from a mixture prepared by adding an organic agent for cutting off near infrared radiation to a resinous binder, and phthalocyanines and metal complexes are known as typical examples of agents for absorbing near infrared radiation.
The known inorganic materials have, however, the property of reflecting or absorbing visible light along with the near infrared radiation which is an important source of solar heat, and are likely to form a surface glittering like a mirror detracting from its appearance, or a surface having an undesirably low transmittance of visible light. When any such inorganic material is used to form a film covering a transparent material on a house, building, or vehicle, it is necessary to ensure that the film be so small in thickness as to be capable of transmitting visible light satisfactorily. It has been necessary to form a film having a very small thickness on the order of 10 nm by using a physical coating method, such as spraying and baking, CVD, sputtering or vacuum vapor deposition. It has been necessary to use a large apparatus, or vacuum equipment resulting in a high film cost which is undesirable from a productivity standpoint, or for the formation of a film covering a large surface. Moreover, the inorganic materials have often tended to make a film of high electrical conductivity which reflects radio waves to be received by e.g. a portable telephone, a television receiver or a car navigation system having an aerial installed within a vehicle, or causes jamming in a neighboring area.
The known organic agents have been fatally low in weatherability because of heavy deterioration by heat or humidity. Moreover, a film having a high transmittance of visible light has little power to cut off heat radiation, while a film having high power to cut off heat radiation has only a low transmittance of visible light.
Antimony-containing tin oxide (hereinafter referred to as ATO) and tin-containing indium oxide (hereinafter referred to as ITO) are known as materials having relatively low absorption or reflection of visible light and high transparency to the human eye, but as they have only low power to cut off solar radiation per unit mass, a film which can cut off heat radiation effectively has had to be prepared from a large amount of material, and has, therefore, been very expensive. Moreover, ATO has been unsatisfactory, as its free electron concentration is too low to enable any satisfactory cutoff of near infrared radiation. A film formed by a physical coating method has been so high in electrical conductivity as to reflect or interfere with radio waves.